Right now, we know that Cassie Earring drops from KS30 Atropos in Horlais Peak. Serket Ring drops from KS30 Lachesis in Waughroon Shrine. By the pattern, assuming that both location and orb are taken into account, the last unaccounted-for item (Speed Belt) should drop from KS30 in Balga's Dais, from the Clotho Orb BC.
This leads us to the Season's Greetings theory. However, Cactor Sauve and Come Into My Parlor are also Clotho from different locations than the first two items. They may be worth looking into.
However, it's possible that SE was only using the location variable, or only the orb variable. There are five potential KS30 locations, and three potential KS30 orbs.
Going by the different-orbs theory, the five Clotho BCs fit the pattern:
Come Into My Parlor - Qu'Bia Arena
Cactor Sauve - Chamber of Oracles
Double Dragonian - Horlais Peak
Copycat - Waughroon Shrine
Seasons Greetings - Balga's Dais
Going by the different-locations theory, nine BCs fit:
Balga's Dais
Seasons Greetings (Clotho)
Royale Ramble (Lachesis)
Moa Constrictors (Atropos)
Qu'Bia Arena
Come Into My Parlor (Clotho)
E-vase-ive Action (Lachesis)
Infernal Swarm (Atropos)
Chamber of Oracles
Cactor Sauve (Clotho)
Eye of the Storm (Lachesis)
The Scarlet King (Atropos)
I personally see the different-orbs theory holding more water (3 items, 3 orbs) but the fact that there are three Beastman strongholds also seems possible. So if I had to guess, I would think it was in a Clotho KS30.
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